JPS59176501A - Boiler tube - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a boiler tube, and particularly to a boiler tube having a double pipe structure.

[Prior art]

Conventionally, as a boiler tube, the steam temperature inside the tube is approximately 570C or less, and the temperature of the outer wall surface of the tube on the combustion gas side is 6C.
5US304 on the source side on the relatively low temperature side of 00 to 650C
, 5US321 and 8US347 are used. However, in recent years, the steam temperature has been increased to 600C and further to 65C for reasons such as power generation efficiency.
There is a trend towards increasing the temperature and pressure to around 0C. Therefore, the temperature of the steam inside the boiler tube and the temperature of the outer wall of the tube must be increased by about 30 to 100 tZ" compared to the conventional method. However, in general, the temperature of one steam is 570 tZ".
In austenitic stainless steels used at relatively high temperatures near C, steam oxidation significantly increases when the steam temperature exceeds about 600 C, and corrosion of the outer wall surface by combustion gas is accelerated.

As such high-temperature tubes, there is a boiler tube with a double-tube structure having an inner tube with excellent steam oxidation resistance and an outer tube with excellent high-temperature corrosion resistance.

In recent years, however, efforts have been made to increase the steam temperature in coal-fired boilers as well. The high-temperature corrosion caused by coal combustion gas in this coal-fired boiler is different from that caused by conventional heavy oil combustion gas, and even materials that are excellent against high-temperature corrosion caused by heavy oil combustion gas do not necessarily suffer from high-temperature corrosion caused by coal combustion gas. The development of boiler tubes with excellent steam oxidation resistance, high-temperature strength, and high-temperature corrosion resistance is expected.

[Purpose of the invention]

An object of the present invention is to provide a boiler tube that is excellent in steam oxidation resistance and high-temperature strength, and also has high corrosion resistance against coal combustion gas.

[Structure of the invention]

In order to achieve this object, the present invention provides a boiler tube with a double tube structure consisting of an inner tube and an outer tube, in which the inner tube has a carbon content of 0.02 to 0.15% by weight.

191Q, 5-3.5%, Mn 2% or less, Ni2O-3
5%, Cr 20.5-27%, Mo 0.5-3%.

Nb 1% or less and Ti 0.5% or less, respectively.

The boiler tube contains at least one strain of W and B, has a composition with the remainder being PE, has a substantially entirely austenitic structure, and has a high corrosion resistance in the outer tube as well as the inner tube. It is something to do. That is, the boiler tube of the present invention adopts a double-tube structure consisting of an inner tube that has excellent steam oxidation resistance and high-temperature strength, and an outer tube that also has high corrosion resistance. .

Next, the reason for limiting the components in the inner tube will be explained first.

Note that % indicates weight %.

C: When contained at 0.02% or more, C combines with carbide-forming elements such as Mo5Nl) to form intragranular carbides and increases high-temperature creep strength, but when the content exceeds 0.15%, it forms grain boundaries. 0.02 to 0.15%, since precipitation of carbides reduces corrosion resistance and adversely affects workability and weldability.
And so. 49K O, 03-0.08% is preferred.

8i: When Sj is added in an amount of 0.5% or more, corrosion resistance is significantly improved, but if the amount exceeds 3.5%, manufacturability and processability are significantly impaired, making it extremely difficult to manufacture pipes, and ferrite 0.5 to 3 to precipitate the phase.
It was set at 5%.

Cr: If Cr is 20.5% or more, it increases the steam oxidation resistance, but if it exceeds 27%, high temperature strength is lost, so 20.5 to 2
It was set at 7%. Particularly preferred is 22 to 26%.

MO: 0.5 to 3% MO strengthens the austenite matrix without adversely affecting steam oxidation, precipitates as carbides in some parts, increases high-temperature strength, and strengthens grain boundaries.

On the other hand, if it exceeds 3%, ferrite is generated, which facilitates precipitation of the sigma phase and significantly reduces workability, so it is set at 0.5 to 3%. Particularly preferred is 1 to 2.5%.

Nb: When Nb. is 1% or less, it precipitates as a carbide and increases high temperature strength and ductility. Especially 0.
1 to 0.5% is preferred.

Ni: Ni coexists with Cr to enhance plastic workability and requires a content of 20% or more to stabilize the austenitic structure. However, if the Cr content exceeds 35%, the columnar crystals will become coarse and cause cracking during plastic working, so it is set at 20 to 35%. In particular, 29 to 33% is preferable.

Ti, W: Ti and W precipitate carbides in the matrix to improve high-temperature ductility, and refine grains to improve corrosion resistance. However, if it exceeds 0.5%, weldability deteriorates and welding defects may occur, so it is set at 0.5% or less.

Particularly preferred is 0.1 to 0.4%.

B: 0.5% or less of B is effective in strengthening grain boundaries and obtaining high-temperature ductility. If the Naori/C ratio is 0.2 or more, it will affect weldability and workability, so it is preferably 0.002 to 0.03% in relation to the C content.

The PE-based heat-resistant alloy used for this inner tube has a 20.5 to 27%
MO on Cr-20-35%Ni steel.

This is the addition of Nb and st. In particular, when compared with 1ncoloy800, a Fe-based heat-resistant alloy, Ti
, At is removed, and Mo, Nb, and Si are added in combination. In addition, it is necessary to improve the corrosion resistance of the inner tube against water vapor compared to the conventional 5US304゜5U8321 boiler tube material (CCR content: 18 to 20%), and for this purpose, it is necessary to increase the Cr content to 20.5% or more. It becomes necessary. However, 600 to 65
It is difficult to obtain sufficient corrosion resistance against 0C steam, and Si
Must be added at least 0.5%. Since both Cr and St are ferrite-forming elements, Cr accompanying Si addition
It is preferable to increase the equivalent weight by decreasing the amount of Cr or increasing the amount of Ni equivalent to improve plastic workability.

Naturally, as the temperature rises, high-temperature strength also becomes important. Ni
- In Cr-based steels, Ni and Cr equivalents have important effects on stabilization of the austenite structure and plastic workability. As shown in Figure 1, by increasing the Ni equivalent by 2 to 15% above the straight line indicating the austenite region, a stable structure can be obtained, good ductility can be obtained, and a seamless pipe without cracks can be produced by plastic working. It can be outfitted.

Ni and Cr equivalents are expressed by the following formula. ()
The percentages are by weight.

N1fi amount=Nie/a+c30Xc%)+CO,sx
1Mne9:) Cr equivalent = C'+MO? J+(1,5
X81%) + (:o, sxN direction: Ir1 in the 1 inner tube
The Cr content is preferably 24 to 33%, and the Ni equivalent is preferably 32 to 35%.

On the other hand, the present invention has a double-tube structure as described above, and the outer tube has higher corrosion resistance than the inner tube, but since the outer tube is also fixed by welding etc., it has a high temperature strength. A material with high weldability is preferable. A preferred composition for this outer tube is as follows. That is, C0.02~0.2%
, Si 3.5% or less, M n 2% or less, N133-4
3% or less, 35 to 40% Cr, and the remainder pe. Further, K may be further added to include 0.5 to 3% of MO and 1% or less of Nb. In addition to this yIO and Nb residue, or together with Mow Nb, 0.5% or less of W, respectively.

T1. B may also be included.

Next, we will discuss the reason for limiting the ingredients in this outer tube (9) Bell.

C: 0.02% or more of C is Mo, Nb, W.

Combines with carbide-forming elements of TM to form intragranular carbides,
It increases the high temperature creep strength, but if the content exceeds 0.2%, it will reduce the precipitation of grain boundary carbides and have an adverse effect on workability and weldability, so a lower content is more effective. Especially KO, 03 to 0.08% is preferable.

Cr: Cr is extremely effective against high-temperature corrosion caused by coal combustion gas, but if it exceeds 40%, the effect will not improve, so 35 to 40% is preferable.

Mo: 0.5-3% MO strengthens the austenite matrix without affecting high-temperature corrosion by coal combustion gas, 35-40Cr-Ni3-37%
It precipitates in steel as a carbide, increasing high-temperature strength and strengthening grain boundaries. If it exceeds 3%, it will adversely affect workability! Therefore, 1 to 2% is particularly preferable.

Nb: 1% or less Nb precipitates as a carbide and (10
) Increases high temperature strength and improves ductility.

If it exceeds 1%, workability and weldability will be significantly reduced, so 0.1 to 065% is particularly preferable.

W, Ti: W, Ti precipitates carbides in the matrix without reducing corrosion resistance and improves high-temperature ductility. However, in consideration of weldability, the content is preferably 0.5% or less. Particularly preferred is 0.1 to 0.4%.

B: B strengthens grain boundaries and imparts high-temperature ductility, but does not affect corrosion resistance. In addition, in relation to weldability and the amount of C, it is preferable that the content be 0.03% or less, with a wrinkle of about 0.02 to 0.03%.

Ni:Nt must be 30% or more in order to coexist with Cr, improve plastic workability, and maintain a stable austenite structure. The steel used for this outer tube has a carbon content of 35-40%.
r amount and Mo, Nb.

Since it contains ferrite-forming elements such as W, it is necessary to control the Ni equivalent amount as follows (11) in order to maintain a stable austenite phase. However, Ni combines with S in coal combustion gas to form sulfides, which affects corrosion resistance, so the upper limit has been set to 3.
It is preferable to set it to 7%.

From the above composition, the steel material for the outer tube has a Cr equivalent of 38
-43%, and the Ni equivalent is preferably 35-40%.

In the steel material of this outer tube, % M O + N b +
Corrosion resistance and high-temperature strength are improved by including W*Tt, B, and si, but when ferrite is mixed in the austenite structure, embrittlement occurs due to precipitation of sigma phase at high temperatures. In order to avoid this, it is preferable to set the Ni equivalent to 2 to 15 higher than the ferrite formation curve shown in FIG.

Further, the outer tube having the above-mentioned composition range has excellent high-temperature strength, but this also has the effect that the wall 11 of the inner tube can be made smaller.

In the present invention, the thickness of the inner tube and the outer tube is not particularly limited, but from the viewpoint of maintaining the strength of the tube for the inner tube and the corrosion resistance (12) for the outer tube, It is preferable to select it appropriately.

The boiler tube according to the present invention is manufactured, for example, as follows.

That is, a billet for the outer tube and a billet for the inner tube are used to form a double tube billet, which is heated and hot extruded to metallurgically join the inner tube and the outer tube. After that, it is made into a product through processes such as cooling, cold drawing, heat treatment, and tube cutting.

[Embodiments of the invention]

Table 1 shows the chemical components (% by weight) of the inner tube used in this example. The inner tube sample was heat-treated and then ground to obtain a final surface of 8
After finishing with 00 type merry paper, it was subjected to a test. As for heat treatment, comparative steels 11 and 2. 8U8304 and 5US321
is 1 (150t? x 30 minutes of water cooling, compared to steel 3)
coloy800 and invention steel 1) 7 is 1150CX3
After heating for 0 minutes, aqueous solution treatment was performed.

FIG. 2 shows the relationship between temperature and scale formation thickness when a corrosion test was conducted for 1000 hours in steam at 500 to 700C. As is clear from the figure, the steel of the present invention (13) shows a tendency to improve steam oxidation resistance as the amount of iist increases;
Compared to No. 21 steel, it exhibits steam oxidation resistance that is approximately three times higher.

Inventive steel and conventional steel (comparative steel 1.2), especially as the temperature increases
There is a tendency for the difference in corrosion resistance to increase. Again, same F
, ·, Incoloy 800, a comparative steel that is a base alloy
It can be seen that the corrosion resistance is better.

The results of comparing the creep rupture strength and ductility of the steels of the present invention at 650C, 51, +2.0 and 0 hours are shown.

InGo1oy800 has a creep rupture strength of approximately 14 to 9.
/w” and the ductility is 20%, whereas the steel of the present invention has a creep rupture strength of 16 to 20 Kti/mF and a ductility of 22 to 20%.
At 26% F, it has high temperature strength and ductility equivalent to or higher than Incoloy 800.

Table 3 shows the chemical composition (% by weight) of 1. The outer tube sample was heat treated and then ground, and the final surface was finished with 800mm merry paper before being subjected to testing.

As a comparative steel, 5US304.25Cr-2ONi steel (
14) 5US310.50Cr-5ONi steel Jncone1
671 was used. In addition, the solution treatment was performed on Comparative Steel 1, 5US.
304 is 1050t: ' water cooled after holding for 30 minutes, comparison steel 4
5US310 was held at 1150C for 30 minutes and then cooled with water, comparison steel 5 was cone 1671, and the invention steel was 1200.
After holding at C for 30 minutes, the mixture was cooled with water.

(15) (16) Table 2 (17) (18) Figure 3 shows the test piece containing 41% Kz S 04-34% Na.
zSO4-25%Fezes was applied and 1%S02-5 t was applied, corresponding to the coal combustion exhaust gas composition of 750t:'
- The relationship between corrosion weight loss and Cr amount when corroded in 15% Cot-residual Nz (Pt catalyst) gas for 100 hours is shown.

It is known that the steel of the present invention, which has a higher Cr content than Comparative Steels 1 and 4, has excellent corrosion resistance. Also, the invention steel! :]Cr
Compared to comparative steel 5, jnconel 671 K, which has a high 1i, it is at the same level and is known to have very excellent corrosion resistance.

Table 4 shows the results of comparing the creep rupture strength at 750C for 1000 hours with Comparative Steel 5, 1ficOne l 671.

As can be seen from the table, the steel of the present invention is ncOne167
It is known that the creep rupture strength is superior to that of No. 1.

(19) Table 4 shows the NI of the inner and outer steel of the tube as a double pipe of the present invention.
The equivalent weight and Cr equivalent area are shown. It is clear that the steel of the invention does not crack during the manufacture of seamless steel pipes by extrusion or the Mannesmann process.

〔Effect of the invention〕

As described above, the double tube of the present invention has remarkable high-temperature corrosion resistance compared to conventional steel, and is highly resistant to corrosion on the outer wall of the tube, which is a problem in coal-fired boilers, and steam oxidation, which is a problem when the steam temperature on the inner surface of the tube is increased. (20) It has a remarkable effect of increasing power generation efficiency especially as a boiler tube for power generation plants.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between Cr equivalent and Ni equivalent, Figure 2 is a diagram showing the relationship between Cr equivalent and Ni equivalent.
The figure is a diagram showing the relationship between scale generation thickness and heating temperature.
Figure 3 shows corrosion loss due to high temperature corrosion (21) 2-6-2 Otemachi, Chiyoda-ku, Tokyo

Claims (1)

[Claims] 1. In a boiler tube with a double pipe structure consisting of a circular pipe and an outer pipe, the inner pipe contains CO, -02 to 0.15% by weight.
, 8i0. s ~3.5%, Mn 2% or less, Ni2O ~
35%, Cr2O, 5-27%, λ400.5-3%,
Nb 1% or less and Tt, B, w each 0.5% or less
What is claimed is: 1. A boiler tube comprising at least one of the above, with the remainder being pe, having a substantially entirely austenitic structure, and having a high corrosion resistance in the outer tube as well as in the inner tube. 2. The outer tube has a weight percentage of C0.02~0.2%, Si3.
5% or less, Mn 2% or less, Ni 33-43%. The boiler tube according to claim 1, characterized in that it contains 35 to 40% of Cr, the balance is pe, and has substantially all austenite structure. 3. The outer tube further contains Mo 0.5-3% by weight, Nb
The boiler tube according to claim 2, characterized in that it contains 1% or less. 4. The outer tube further has a Wlo of 0.5% or less in weight%, 5
% or less of 'l'i, at least 1 of 0.03% or less of B
The boiler tube according to claim 2 or 3, characterized in that it contains seeds. 5. The boiler according to any one of claims 1 to 4, wherein the Ni equivalent of the inner tube is 2 to 15 higher than the straight line indicating the lower limit of the total austenite region in FIG. tube. 6. The Cr equivalent of the inner tube is 24 to 33 and the Nt equivalent is 22 to 33.
38, outer tube (DCr equivalent is 36 to 44 and Ni equivalent is 3
9 to 50, the boiler tube according to any one of claims 1 to 5.